Steel sheet products are treated to secure corrosion resistance, rustproofing, coating adhesion, and other characteristics by treating the surface of the steel sheet, or the surface of the steel sheet after being plated by Sn, Zn, Ni, etc., by cathode electrolysis in a treatment solution containing hexavalent Cr (electrolytic Cr acid treatment) so as to form a chromate coating comprised of Cr oxide or Cr metal and Cr oxide.
For example, the Sn-plated steel sheet (tinplate) used as steel sheet for containers and TFS (tin free steel) not using Sn are given corrosion resistance, coating adhesion, film adhesion, and discoloration resistance by giving their outermost surfaces a chromate coating.
In recent years, interest in the environment has been rising and restrictions on the use of hexavalent Cr are being considered. Therefore, the method of applying a chemically treated coating made of a Zr compound as a new coating to take the place of chromate coatings while using existing electrolytic Cr acid treatment facilities has been proposed.
For example, PLT 1 proposes “a method of surface treatment of tin-plated steel sheet and tin-plated cans”, while PLT 2 proposes “a method of surface treatment of a tin- or tin-alloy plated steel material”.
In these inventions, the method is disclosed of treatment by cathode electrolysis in a treatment solution containing Zr fluoride or a Zr compound using a Pt or other insoluble anode as a counter electrode so as to obtain a Zr-based chemically treated coating with superior performance.